In a modern construction process, especially in the construction of commercial buildings, self-drilling, self-tapping screws are needed that are capable of extruding through metal sheets, such as corrugated steel roof deck or steel studs, with relative ease to facilitate a rapid completion of construction projects. In addition, the screws must have high pullout values that is, greater holding power, so as to prevent them from being readily pulled out. For example, screws with the above-described qualities are needed in installations of commercial and factory roofs having a flat roof design. The roof installation involves placing a layer of insulation on the generally corrugated steel roof deck, and then covering the insulation with a single ply thermoplastic roofing membrane so as to protect the underlining roofing components against the elements. A common method of securing the roofing membrane to the roof is to attach the edges of the membrane to the deck using an elongated screw passing through a plate or washer placed on the membrane. It has been found that, because the membrane is exposed to the elements, it may at times be subject to high velocity winds blowing across the roof, which cause uplift forces. These forces cause the membrane to billow upwardly. As this occurs, the membrane tends to pull up and exerts an upward force on one edge of the plate, which causes the screw to eventually work its way out of the roof deck.
A known fastener for drilling through panel material, including a steel panel, is disclosed in commonly assigned U.S. Pat. No. 4,693,654 which is incorporated herein by reference. The screw of the '654 patent has a self-drilling tip formed by a pair of conical segments with each segment having a flat side and a rounded tip and with one segment being longitudinally offset with respect to the other. The screw of the '654 patent also features a quad-lead thread having two crests with a first diameter and two crests with a second larger diameter.
One disadvantage of the fastener disclosed in the U.S. Pat. No. 4,693,654 or of other conventional self-tapping, self-drilling screws of that type is that, especially when applied in a roof plate attachment system of the type described above, the threads are relatively shallow and therefore have a low pull-out value. In other words, screws of this type are more susceptible to the membrane pulling forces described above. Another disadvantage of such screws in the above-identified environment is that the threads stop at the base of the tip, requiring a longer time and more force before the threads catch a pilot hole. Contributing to this problem is the fact that the drill tip is relatively large for the purposes of drilling the pilot hole.
Another known fastener for drilling through a metal panel has a threaded shank portion that decreases in diameter towards a self-drilling tip, and which has a flute for directing metal shavings away from a hole being drilled by the fastener. The disadvantage of this fastener is that the threads stop well before reaching the drilling tip, requiring, as in the fastener disclosed in the U.S. Pat. No. 4,693,654, a longer time and more force before the threads catch the opening made by the drill tip.